1. Field of the Invention
The present invention relates to a method for inhibiting gravel pack and formation sandstone dissolution during steam or water injection and, more particularly, to a method for protecting the gravel pack liner used in oil wells being subjected to increased recovery techniques using steam as a driving force as well as a method for protection of the face of the producing zone involved in these techniques.
2. Description of the Prior Art
In the production of crude oil from wells, various techniques have been used to enhance recovery past that which is possible through normal well pressures. These techniques, which are collectively referred to as "secondary recovery", include water flooding, steam stimulation and fire flooding. The success of these techniques, together with the need to increase domestic oil production, has led to the expanded use thereof in the oil fields.
Steam stimulation and steam flooding are techniques generally used in reservoirs of high-viscosity oil. The techniques involve injection into the well of steam of high temperature (approximately 500.degree. F.) in cycles of thousands of cubic meters at a time. The quality of this steam generally ranges from 60-80%, meaning that large quantities of liquid water are concurrently injected into the well bore with the steam.
A typical oil well consists of a casing which lines the inside surface of the well bore and a length of tubing which extends downwardly through the casing. The casing serves to protect the tubing in the event of damage to the latter. Sucker rods extend through the tubing and terminate in a pump which reciprocates in the tubing and forces the oil upwardly therethrough. The lower end of the tubing extends into the oil zone and has perforations therein through which the oil flows thereinto.
Many wells which are subjected to steam stimulation have the lower end of the tubing filled with a gravel pack. A liner is positioned on top of the gravel pack and serves as a seat for the oil pump. The function of the gravel pack is to filter and prevent sand from being produced with the well fluid. The sand is erosive and if not filtered, would damage the pump. The gravel used to pack the well consists of granular sand grains. This material is principally quartz or silica.
Silica has a very low solubility in water at neutral pH and low temperatures, but this solubility rises sharply as temperature and pH are increased. For pH values above 11.0 and temperatures above 177.degree. C., the dissolution rates are orders of magnitude higher than at neutral/ambient conditions.
The liquid water produced in a steam generator generally has a pH in excess of 11.0. Coupled with the high liquid temperatures, the fluid is capable of rapidly dissolving the gravel pack. In the event of failure of the gravel pack, the well begins to produce sand with the eventual shut-down of the well.
Not only is the rate of silica dissolution quite rapid, but the water in the well becomes saturated within a short distance from the point at which the fluid contacts the surface of the silica. This is significant in that the dissolution of silica tends to be localized rather than diffused over a wide area of the zone, resulting in the face of the zone receding significantly.
In addition to the dissolution of the gravel pack due to the large quantities of water injected, there is a danger of the face of the sandstone zone being eroded. If this occurs to a sizeable extent, formation caving in and even tubing or casing collapse could result, resulting in the loss of the well.
Still further, these large silica losses at the well bore may precipitate out as the fluid reaches supersaturated conditions as it passes through the zone. The precipitation of the silica in the zone may result in loss of zone permeability and a resultant shutin.
The costs resulting from such well failures are imposing. Recently, one large oil producer estimated a well failure rate of 34% due to failure of gravel packing or zone related problems due to steaming. The approximate cost of reworking a well presently runs over $40,000. Reducing the cycle by even one day would realize significant savings.
Dissolution of the gravel pack has been shown to be primarily a function of the pH and temperature of the injected liquid-phase water. Consequently, prior attempts at solution of the problem have focused on these aspects. For example, by keeping the pH of the injected hot water below 9.5, gravel pack dissolution can be decreased sharply. This may be accomplished by (1) selection of feedwaters having low HCO.sub.3.sup.- ion concentrations (&lt;10 mg HCO.sub.3.sup.- /L), (2) treating the feedwater with HCl to yield the desired effluent pH, or (3) using a total deionizer to remove both cations and anions from the feedwater.
With regard to the first proposed solution, selection of feedwaters is often impractical as the large quantities of water used are not available from a choice of sources. With regard to the second proposed solution, the use of HCl to neutralize the bicarbonate alkalinity suffers from considerations of cost as well as feasibility of the method. That is, addition of too much acid will cause severe corrosion of the steam generator and too little will result in insufficient depression of the pH to alleviate silica loss. With regard to the third proposed solution, the cost of a total deionizer is prohibitive, both in terms of capital costs as well as daily maintenance costs.
Even if any or all of these methods were satisfactory in reducing the pH to a level at which the gravel pack dissolution was abated, studies have shown that the sandstones of the zone are comprised of materials which will dissolve at these high temperatures even at neutral pH. As a result, regardless of gravel pack protection by lowering the pH of the hot water which accompanies the steam, large volumes of well bore surface may be eroded.